Medical treatment of several illnesses requires continuous drug infusion into various body compartments, such as subcutaneous and intra-venous injections. Diabetes mellitus patients, for example, require administration of varying amounts of insulin throughout the day to control their blood glucose levels. In recent years, ambulatory portable insulin infusion pumps have emerged as a superior alternative to multiple daily syringe injections of insulin. These pumps, which deliver insulin at a continuous basal rate as well as in bolus volumes, were developed to liberate patients from repeated self-administered injections, and allow them to maintain a near-normal daily routine. Both basal and bolus volumes must be delivered in precise doses, according to individual prescription, since an overdose or under-dose of insulin could be fatal. Therefore, insulin injection pumps must feature high reliability, totally preventing delivery of any unintentional insulin excess.
Several ambulatory insulin infusion devices are currently available on the market. Mostly, these devices have two portions: a reusable portion that contains a dispenser, a controller and electronics, and a disposable portion that contains a syringe-type reservoir, a needle assembly with a cannula and a penetrating member, and fluid delivery tube. Usually, the patient fills the reservoir with insulin, attaches the needle and the delivery tube to the exit port of the reservoir, and then inserts the reservoir into the pump housing. After purging air out of the reservoir, tube and needle, the patient inserts the needle assembly, penetrating member and cannula, at a selected location on the body, and withdraws the penetrating member. To avoid irritation and infection, the subcutaneous cannula should be replaced and discarded after 2-3 days, together with the empty reservoir. Examples of first generation disposable syringe-type reservoir and tubes were disclosed in U.S. Pat. No. 3,631,847 to Hobbs, U.S. Pat. No. 3,771,694 to Kaminski, U.S. Pat. No. 4,657,486 to Stempfle, and U.S. Pat. No. 4,544,369 to Skakoon. The driving mechanism of these devices is a screw thread driven plunger controlling the programmed movement of a syringe piston.
Other dispensing mechanisms have been also discussed, including peristaltic positive displacement pumps, in U.S. Pat. No. 4,498,843 to Schneider and U.S. Pat. No. 4,715,786 to Wolff. These devices represent an improvement over multiple daily injections, but nevertheless, they all suffer from several drawbacks. The main drawback is the large size and the weight of the device, caused by the configuration and the relatively large size of the driving mechanism of the syringe and the piston. This relatively bulky device has to be carried in a patient's pocket or attached to the belt. Consequently, the fluid delivery tube is long, usually longer than 60 cm, in order to permit needle insertion at remote sites of the body. These uncomfortable bulky devices with a long tube are rejected by the majority of diabetic insulin users, since they disturb regular activities, such as sleeping and swimming. Further, the effect of the image projected on the teenagers' body is unacceptable. In addition, the delivery tube excludes some optional remote insertion sites, like buttocks, arms and legs.
To avoid the consequences of long delivery tube, a new concept, of second generation pump, was proposed. This concept includes a remote controlled skin adherable device with a housing having a bottom surface adapted to contact patient's skin, a reservoir disposed within the housing, and an injection needle adapted to communicate with the reservoir. These skin adherable devices should be disposed every 2-3 days similarly to available pump infusion sets. These devices were disclosed at least in U.S. Pat. No. 5,957,895 to Sage, U.S. Pat. No. 6,589,229 to Connelly, and U.S. Pat. No. 6,740,059 to Flaherty. Additional configurations of skin adherable pumps were disclosed in U.S. Pat. No. 6,723,072 to Flaherty and U.S. Pat. No. 6,485,461 to Mason. These devices also have several limitations: they are bulky and expensive, their high selling price is due to the high production and accessory costs, and the user must discard the entire device every 2-3 days, including relatively expensive components, such as driving mechanism and other electronics.
Further, a syringe-type reservoir has at least the following limitations:                It has a round cross section, which increases the device thickness because of inefficient use of space.        Relatively high rotational momentum is required for displacement of the syringe due to the high friction associated with the round gasket and the high surface-area ratio of the plunger and outlet port.        It is difficult to achieve highly accurate dispensing rates because minimal plunger movements cause large amounts of dispensed fluid.        
Thus, there is a need for a fluid dispensing device that would avoid the above limitations and disadvantages of conventional units.